When you generate your form to submit a new posting, include a unique serial number as a hidden field, and store it on a database on server side.

When the response is submitted, check for the serial number, and if it's absent or already used, then redirect to the form again. If it hasn't been used, process the form and delete the serial number from your database.

Hmm. That sounds like a good idea for a column. {grin} I've put it in the to-do bucket.

-- Randal L. Schwartz, Perl hacker